1. Field of the Invention
This invention relates to a practical means for allowing greatly increased corona currents to flow between the electrodes of corona discharge devices, particularly where sparkover and/or back corona may be a problem such as in electrostatic precipitation of high resistivity particles entrained in a gas stream. More specifically, the invention is directed to a resistive coating on a passive electrode having a surface layer of at least 0.25 mm thick, in which the coating is substantially homogeneous and has a volume resistivity and dielectric strength within specified ranges for suppressing back corona and preventing sparkover. The invention also can be used to prevent sparking in corona discharges where there is no foreign material on the anode surface.
2. Description of the Prior Art
Standards for emissions of particulate in flue gases issuing from coal fired electrical power station stacks are becoming increasingly more stringent. Current air quality standards require that more than 99% of the fly ash produced by burning coal be removed prior to discharge of the combustion gases from the stack. Thus, the efficiency of particulate collection must increase in proportion to the ash content of the coal. In addition, in an effort to reduce the emissions of certain gaseous pollutants, particularly the sulphur oxides, it has become increasingly necessary to use low sulphur coal in electrical power generating plants.
The electrostatic precipitator is the most commonly used device for the removal of particulate matter produced by coal fired power plants. In a two-stage electrostatic precipitator the particulate-laden gas sequentially passes through separate charging and collecting stages. In the charging stage the gases pass through a corona discharge so that the particulate matter leaving the charger has a negative charge. The charged particles then pass through a low intensity electric field in the collecting stage which causes the particles to migrate toward a collecting electrode where they are deposited and are subsequently removed and disposed of by various techniques. In a single-stage precipitator, particles flowing between a pair of electrodes having a corona current producing electrostatic field extending therebetween are first charged and then migrate toward one of the electrodes where they agglomerate and are subsequently removed. Thus, in a single-stage precipitator both the charging stage and the collecting stage are combined into a single unit. The efficiency of an electrostatic precipitator is determined to a large extent by the magnitude of the charge placed on the particulate matter by the charging stage. The charge magnitude may be increased by increasing the intensity of the electrostatic field producting the corona discharge. The useful maximum intensity of the electrostatic field is limited to a value at which extremely intense back corona or sparkover occurs as the particulate matter builds up on the passive or non-corona emitting electrode. Back corona exists because in operation there will always be some coating of a particulate layer such as fly ash on the surface of the anode. The current flowing to the anode produces a sufficiently high voltage across the fly ash or other particulate layer to cause electrical breakdowns in it. The intense local ionization in the electrical arc in the breakdown channel causes the ejection of products of ionization into the high-intensity corona discharge field resulting in the triggering of a spark. Although back corona effects can be reduced to some extent by such techniques as limiting the thickness of the particulate layer on the passive electrode, electrostatic field intensities achievable with these techniques nevertheless provide limited particle charging. Thereafter, the collection efficiency must be improved by increasing the residence time of the particulate-matter in the electric field during collection either by reducing the speed at which the particulate-laden gases pass through the collection stage, or by increasing the length of the collection stage. However, a decrease in transit speed through the collection stage reduces the capacity of the collection stage, and increasing the size of the collecting electrodes increases the capital cost of such equipment.
The intensity of the electrostatic field at which the charger can operate without back corona and sparkover is lower for higher resistivity particulate matter. Since fly ash resistivity is inversely related to the level of combustible sulphur in coal, the increasing use of low sulphur coals increases the cost of achieving a high collection efficiency since back corona and sparkover problems are increased. Other particulates, such as those generated by cement producers, also have high resistivities which interfere with the operation of precipitators in which they are collected.
Attempts have been made to reduce the incidence of back corona and sparkover in order to increase the intensity of electrostatic fields in ionizers through a number of techniques none of which are entirely satisfactory. Earliest attempts, as described by H. J. White, Industrial Electrostatic Precipitation at page 328, Addison-Wesley 1963, were directed to treating the particulate matter before entering the ionizer. High resistivity particulate matter was generally treated by moisture and acid conditioning. Other techniques attempted to prevent the buildup of a layer of particulate material on the passive electrode such as by employing moving belt electrodes, rotating brushes and various other mechanical devices. These later techniques generally failed since even thin films of particulate matter can produce servere back corona effects if the resistivity of the particulate matter is sufficiently high. However, particulate matter buildup has been successfully prevented to some extent by continuously flushing the passive electrode with a water film. Still another approach attempts to adjust the temperature of the electrodes or the gas upwardly and downwardly in order to shift the temperature of the particulate matter toward a lower resistivity value. However, this technique generally requires a large amount of power to produce the required temperature shifts.
Previous attempts to adjust the electrical characteristics of the passive electrode in order to reduce back corona and sparkover have generally used a collection electrode made of a resistive material having a non-critical resistance. These electrodes, termed "graded resistance" electrodes, inherently functioned as a current limiting series resistance, and they had only a minor effect in reducing sparkover in electrostatic precipitators. The resistance has the effect of producing a large voltage drop within the electrode as current increases at times of abnormal transient conditions thereby lowering the intensity of the electrostatic field across the electrode gap. Previously used "graded resistance" electrodes have generally been large planar slabs of somewhat conducting materials such as cement-asbestos, or of concrete with an imbedded grid of reinforcing steel to facilitate to a crude degree more uniform current collection. The approach simply was not sufficiently effective for broad commercial application. The graded resistance electrodes were not of a specified maintained design resistivity or uniformity of resistivity. In general their resistivity changed significantly with moisture, absorbed chemical content, and electric field in the material. They never were a satisfactory means of limiting sparking. The inadequacy of all electrostatic precipitator electrodes operating as simply current limiting devices is clearly evidenced by the fact that none are used in present-day commercial apparatus. (Resistors now under development with electrostatic precipitators are employed only to limit the fault current to a unit in which there is a sparkover, in order to minimize the momentary lowering of the voltage on all other units of the group supplied by a common rectifier.)
A form of current limiting resistance, also called a "graded resistance", is described in H. J. White, Resistivity Problems in Electrostatic Precipitation, Journal of the Air Pollution Control Association, 24, pages 336-37 (1974). In accordance with this technique a metal plate was coated with a carbon-impregnated plastic having a resistivity between 10.sup.10 and 10.sup.11 ohm-cm as determined by the degree of carbon loading. The description of this approach was never definitive as to the specific volume resistivity, material thickness, dielectric strength or homogeneity at, and close to, the anode surface which would allow the technique to be utilized with a variety of ionizer designs. Also, the article's description of the carbon-impregnated plastic composition of the coating suggests that the outer 0.25 mm of the coating need not be homogeneous to any specific value in order to effectively prevent spark formation and suppress back corona. Instead, the article appears to describe an attempt to insert an appreciable resistance in series with the discharge, with no engineered concept specifying critical design parameters. This is suggested by the statement contained therein that the concept "is by no means new, in that it originated in the 1920's during the early work on electrical precipitators."
In summary, the only mechanism applicable to "graded resistance" technology is that of inserting a series resistance in the discharge circuit in order to reduce the driving voltage thereby throttling total current flow.
Recently, a high intensity ionizer has been developed in which a unique electrode geometry produces a stable, high intensity corona discharge through which the particulate-laden gas passes. This ionizer which is described and claimed in U.S. Pat. No. 4,110,086 charges the particulate matter to a much higher level than is achievable with conventional ionizers utilizing, for example, wire-cylinder or wire-plate geometries. Although the collection efficiency of two-stage electrostatic precipitators can be greatly improved by employing this unique high intensity ionizer as a charging stage, back corona and sparkover has nevertheless been a problem, particularly with very high resistivity particulate matter, as the particulate matter builds up on a metal passive electrode.
In a low pressure gas electrical discharge, as in a flourescent light tube, the energy balances in the discharge is such as to produce operation with the current flowing with low density in a large-diameter column. But the physics of electrical discharges is such that with increase in pressure the discharge diameter decreases at such a rate that the current density increases as the square of the gas density. At atmospheric pressure and ordinary ambient temperatures all electrical discharges inherently contract into the narrowchannel high-current-density low-electric-field form termed an arc, which in transitory form is called a spark. This invention is a basic means for preventing the contraction of a corona discharge into an arc form.
With a negative corona discharge the gas in the near-vicinity of an electric field concentrating cathode is momentarily broken down, causing paths of intense ionization to propagate a small fraction of the distance to the anode. Electrons set free in the intense ionization processes drift toward the anode, usually attaching to molecules to form negative ions before arriving at the anode as a low-density (0.1-10 .mu.A/cm.sup.2) flow of current. The corona discharge is a rapid succession of non-completed discharges in the cathode-anode space, but current to the anode is, in the main, a steady uni-directional current.
In most applications of practical importance it is essential to operate corona discharges with as high intensities as possible without excessive back corona or sparkovers. A critical condition is reached rapidly because the current increases about as the square of the applied voltage. At the critical point there is a sudden local transition from a high-field low-current-density discharge to a low-field high-current-density discharge, i.e. from a glow-type to an arc-type of discharge.